In August 1943, the sales team at Gallimard noticed something odd. The publisher’s new 700-page philosophical tome was selling unexpectedly well. Was it because Jean-Paul Sartre’s thoughts on freedom and responsibility in Being and Nothingness resonated with Parisians enduring Nazi occupation? Not quite. It was because the book weighed exactly one kilogram and so was a perfect substitute for copper weights, which had been sold on the black market or melted down for ammunition.

Agnès Poirier’s engaging romp through the decade in which Paris rose from wartime shame to assert its claims to be world capital of art, philosophy and turtlenecks teems with such vignettes. When Ernest Hemingway arrived in Paris with fellow allied liberators in August 1944, for instance, he parked his Jeep outside 7 Rue des Grands Augustins – Picasso’s wartime studio. No one was in, so he left his impeccably butch calling card – a bucket of grenades, plus a note reading: “To Picasso from Hemingway”. Poirier never tells us what happened to those grenades.

In one of my favourite moments Simone de Beauvoir pauses on the Pont Neuf after a nuit blanche of drinking with Sartre, Arthur Koestler and Albert Camus. Looking sadly into the Seine, she sobs over the tragedy of the human condition. “I do not understand why we do not throw ourselves into the water!” she wails to Sartre who, also crying, replies: “Well, let’s do it!” It would take a heart of stone not to laugh.

But what is the human condition? Sartre defined it shortly after the liberation. “We were never freer,” he wrote, “than during the German occupation. Since the Nazi venom was poisoning our very own thinking, our every free thought was a victory. The circumstances, often atrocious, of our fight allowed us to live openly this torn and unbearable situation one calls the Human Condition.” But, Poirier points out, that freedom was dubiously won. De Beauvoir signed a form denying she was Jewish so she could continue teaching in occupied Paris. While she and Sartre were never freer, Parisian Jews were being rounded up by Parisian cops and murdered in Nazi death camps.

Celebrity collaborators, too, in on-trend if unwitting existentialist fashion, defined their moral characters through what they did rather than what they thought – and later came up with shameless rationalisations. Arletty, star of Marcel Carné’s film Les Enfants du Paradis, justified sleeping with the enemy by mapping her body as semi-autonomous regions. “My heart is French, but my arse is international,” she said. During interrogation by the resistance, actor and playwright Sacha Guitry was asked: “Why did you have dinner with Hermann Göring?” “Out of curiosity,” he witlessly replied.

Poirier, though, risks soft pedalling these evasions and self-delusions since, ardent Parisienne that she is, she wants to tell a love story. In her narrative, everyone who is creatively or intellectually anyone is seduced by Paris. Her enviable cast of characters – not just existentialist philosophers but Samuel Beckett, Alberto Giacometti, Juliette Greco, Jean Cocteau, Simone Signoret, and wave after wave of oversexed, overpaid Americans – are libidinous multi-taskers, overturning bourgeois norms while philosophising, be-bopping, pill-popping and bed-hopping.

In her first book, Touché: A French Woman’s Take on the English, Poirier was unexpectedly captivated by repressed, anti-intellectual, ill-groomed rosbifs, but here she returns to her first love, Paris, and, as happens when you’re besotted, allows herself much poetic licence and novelettish prose. Here are Norman Mailer and his wife on their way to French classes. “Every morning, wearing layers of woollen sweaters, they walked through the Luxembourg Gardens towards the Boulevard Saint-Michel and the Sorbonne, all the while conjugating subjunctive verbs and kissing.” Oh come on. Every morning?

Mailer was one of many Americans bankrolled by the fabulous-sounding GI Bill. That legislation, so far as I understand it, paid for veterans to return to Paris, learn French and get laid. The Americans in Paris who prove most sympathetic in Poirier’s story are black. She traces the experiences of three African Americans – Richard Wright, James Baldwin and Miles Davis – each blindsided by finding romance and creative stimulation away from the US. Poirier, though, doesn’t nail why Paris, today hardly a byword for racial harmony, was then so appealingly colour blind.

She is also, unusual for a Parisienne, philosophy light. If you want to know how Sartre regaled Café de Flore waiters with his “phylogeny recapitulates ontogeny” monologue, try Sarah Bakewell’s recent At the Existentialist Cafe. Poirier is more infectious in her enthusiasms than Bakewell, though – so much so that when grumpy Saul Bellow arrives in town as everyone else is partying like it’s 1949, it’s hard not to share her exasperation. His New World ressentiment for Paris’s charms makes him determined to have no fun.

And yet, in Poirier’s story, Paris seduces even Bellow in the end. She writes: “On the first bright spring day of 1949 the Paris street cleaning system gave Saul Bellow the breakthrough that would lead directly to the Nobel prize in literature.” He had an epiphany while contemplating the waters with which Paris gutters are daily swilled. Those free-flowing rivulets released his writer’s block, helping him realise the “untamed and unabashed” sentences of his first masterpiece, The Adventures of Augie March. That “directly”, though, typifies Poirier’s tendency to overstate how Paris shaped postwar culture.

Only twice did her love letter make me choke on my citron pressé. While I tip the proverbial beret to 1940s Paris for creating circumstances for De Beauvoir to write The Second Sex, I refuse to follow Poirier in taking Brigitte Bardot as De Beauvoir’s feminist sister. It was Bardot, after all, who recently called the #MeToo movement “ridiculous”, adding: “I found it charming when men told me that I was beautiful or I had a nice little backside.”

Throughout, too, Poirier describes Paris as the place where everyone from Sartre to De Gaulle was working out a third way between the twin cold war barbarisms of American consumerism and Stalinist totalitarianism. That idealism, she argues, resulted in the European Union, an outfit conceived by the visionary civil servant Jean Monnet in Paris in the late 40s. That something is awry with this story becomes clear when Poirier invokes Harold Macmillan, who wrote the 1938 book The Middle Way, as intellectual precursor. It’s implausible that the existentialist Marxist Sartre was even then on the same page as Britain’s future Conservative prime minister, still less De Gaulle, France’s future reactionary president.

The EU could be seen not the way Poirier sees it, as Paris-created bulwark, but as sociologist Wolfgang Streeck envisioned it – a deregulation machine exposing its citizens to capitalism gone wild. I would argue that, were Sartre and De Beauvoir alive, they would share Streeck’s view. But let’s not leave Paris without yielding, just a little, to Poirier’s rose-tinted image of its charms. Interviewed for the book, Juliette Greco recalled evenings 60-odd years earlier, strolling with Miles Davis from jazz club to bistro, as their love blossomed. She was white, he black, she had no English, he no French. “I have no idea how we managed,” she laughed. “The miracle of love.” Or the miracle of Paris, which is much the same thing.

•Left Bank is published by Bloomsbury. To order a copy for £21.25 (RRP £25) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.